/**
 * 加法
 * @param a string|number 加数
 * @param b string|number 加数
 * @return string
 */
$.bcAdd = function(a, b){
	let _a = (a+='').charAt(0)==='-',
		_b = (b+='').charAt(0)==='-';
	if(_a!==_b){
		if(_a) return $.bcSubDot(b, a.slice(1));
		else return $.bcSubDot(a, b.slice(1));
	}
	if(!_a) return $.bcAddDot(a,b);
	let x = $.bcAddDot(a.slice(1),b.slice(1));
	return x==='0' ? '0' : '-'+x;
};
